Re: [PATCH v12 5/6] rust: enable `clippy::cast_lossless` lint

[Tamir Duberstein <tamird@gmail.com>]

On Sun, Jun 15, 2025 at 4:55 PM Tamir Duberstein <tamird@gmail.com> wrote:
>
> Before Rust 1.29.0, Clippy introduced the `cast_lossless` lint [1]:
>
> > Rust’s `as` keyword will perform many kinds of conversions, including
> > silently lossy conversions. Conversion functions such as `i32::from`
> > will only perform lossless conversions. Using the conversion functions
> > prevents conversions from becoming silently lossy if the input types
> > ever change, and makes it clear for people reading the code that the
> > conversion is lossless.
>
> While this doesn't eliminate unchecked `as` conversions, it makes such
> conversions easier to scrutinize.  It also has the slight benefit of
> removing a degree of freedom on which to bikeshed. Thus apply the
> changes and enable the lint -- no functional change intended.
>
> Link: https://rust-lang.github.io/rust-clippy/master/index.html#cast_lossless [1]
> Suggested-by: Benno Lossin <benno.lossin@proton.me>
> Link: https://lore.kernel.org/all/D8ORTXSUTKGL.1KOJAGBM8F8TN@proton.me/
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Tamir Duberstein <tamird@gmail.com>

@Danilo Krummrich could you please have a look for drm and nova?

> ---
>  Makefile                             | 1 +
>  drivers/gpu/drm/drm_panic_qr.rs      | 4 ++--
>  drivers/gpu/nova-core/regs.rs        | 2 +-
>  drivers/gpu/nova-core/regs/macros.rs | 2 +-
>  rust/bindings/lib.rs                 | 1 +
>  rust/kernel/net/phy.rs               | 4 ++--
>  rust/uapi/lib.rs                     | 1 +
>  7 files changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 0ba22c361de8..29cf39be14de 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -481,6 +481,7 @@ export rust_common_flags := --edition=2021 \
>                             -Wclippy::all \
>                             -Wclippy::as_ptr_cast_mut \
>                             -Wclippy::as_underscore \
> +                           -Wclippy::cast_lossless \
>                             -Wclippy::ignored_unit_patterns \
>                             -Wclippy::mut_mut \
>                             -Wclippy::needless_bitwise_bool \
> diff --git a/drivers/gpu/drm/drm_panic_qr.rs b/drivers/gpu/drm/drm_panic_qr.rs
> index dd55b1cb764d..6b59d19ab631 100644
> --- a/drivers/gpu/drm/drm_panic_qr.rs
> +++ b/drivers/gpu/drm/drm_panic_qr.rs
> @@ -404,7 +404,7 @@ fn pop3(&mut self) -> Option<(u16, usize)> {
>              let mut out = 0;
>              let mut exp = 1;
>              for i in 0..poplen {
> -                out += self.decimals[self.len + i] as u16 * exp;
> +                out += u16::from(self.decimals[self.len + i]) * exp;
>                  exp *= 10;
>              }
>              Some((out, NUM_CHARS_BITS[poplen]))
> @@ -425,7 +425,7 @@ fn next(&mut self) -> Option<Self::Item> {
>          match self.segment {
>              Segment::Binary(data) => {
>                  if self.offset < data.len() {
> -                    let byte = data[self.offset] as u16;
> +                    let byte = u16::from(data[self.offset]);
>                      self.offset += 1;
>                      Some((byte, 8))
>                  } else {
> diff --git a/drivers/gpu/nova-core/regs.rs b/drivers/gpu/nova-core/regs.rs
> index 5a1273230306..c1cb6d4c49ee 100644
> --- a/drivers/gpu/nova-core/regs.rs
> +++ b/drivers/gpu/nova-core/regs.rs
> @@ -32,7 +32,7 @@ pub(crate) fn architecture(self) -> Result<Architecture> {
>      pub(crate) fn chipset(self) -> Result<Chipset> {
>          self.architecture()
>              .map(|arch| {
> -                ((arch as u32) << Self::IMPLEMENTATION.len()) | self.implementation() as u32
> +                ((arch as u32) << Self::IMPLEMENTATION.len()) | u32::from(self.implementation())
>              })
>              .and_then(Chipset::try_from)
>      }
> diff --git a/drivers/gpu/nova-core/regs/macros.rs b/drivers/gpu/nova-core/regs/macros.rs
> index 7ecc70efb3cd..6851af8b5885 100644
> --- a/drivers/gpu/nova-core/regs/macros.rs
> +++ b/drivers/gpu/nova-core/regs/macros.rs
> @@ -264,7 +264,7 @@ pub(crate) fn $field(self) -> $res_type {
>          pub(crate) fn [<set_ $field>](mut self, value: $to_type) -> Self {
>              const MASK: u32 = $name::[<$field:upper _MASK>];
>              const SHIFT: u32 = $name::[<$field:upper _SHIFT>];
> -            let value = ((value as u32) << SHIFT) & MASK;
> +            let value = (u32::from(value) << SHIFT) & MASK;
>              self.0 = (self.0 & !MASK) | value;
>
>              self
> diff --git a/rust/bindings/lib.rs b/rust/bindings/lib.rs
> index 81b6c7aa4916..7631c9f6708d 100644
> --- a/rust/bindings/lib.rs
> +++ b/rust/bindings/lib.rs
> @@ -25,6 +25,7 @@
>  )]
>
>  #[allow(dead_code)]
> +#[allow(clippy::cast_lossless)]
>  #[allow(clippy::ptr_as_ptr)]
>  #[allow(clippy::undocumented_unsafe_blocks)]
>  #[cfg_attr(CONFIG_RUSTC_HAS_UNNECESSARY_TRANSMUTES, allow(unnecessary_transmutes))]
> diff --git a/rust/kernel/net/phy.rs b/rust/kernel/net/phy.rs
> index 32ea43ece646..65ac4d59ad77 100644
> --- a/rust/kernel/net/phy.rs
> +++ b/rust/kernel/net/phy.rs
> @@ -142,7 +142,7 @@ pub fn is_autoneg_enabled(&self) -> bool {
>          // SAFETY: The struct invariant ensures that we may access
>          // this field without additional synchronization.
>          let bit_field = unsafe { &(*self.0.get())._bitfield_1 };
> -        bit_field.get(13, 1) == bindings::AUTONEG_ENABLE as u64
> +        bit_field.get(13, 1) == u64::from(bindings::AUTONEG_ENABLE)
>      }
>
>      /// Gets the current auto-negotiation state.
> @@ -427,7 +427,7 @@ impl<T: Driver> Adapter<T> {
>          // where we hold `phy_device->lock`, so the accessors on
>          // `Device` are okay to call.
>          let dev = unsafe { Device::from_raw(phydev) };
> -        T::match_phy_device(dev) as i32
> +        T::match_phy_device(dev).into()
>      }
>
>      /// # Safety
> diff --git a/rust/uapi/lib.rs b/rust/uapi/lib.rs
> index e79a1f49f055..08e68ebef606 100644
> --- a/rust/uapi/lib.rs
> +++ b/rust/uapi/lib.rs
> @@ -14,6 +14,7 @@
>  #![cfg_attr(test, allow(unsafe_op_in_unsafe_fn))]
>  #![allow(
>      clippy::all,
> +    clippy::cast_lossless,
>      clippy::ptr_as_ptr,
>      clippy::undocumented_unsafe_blocks,
>      dead_code,
>
> --
> 2.49.0
>